Nitinol or NiTi or TiNi is an alloy of titanium nickel that undergoes an energetic crystalline phase change at near-ambient temperatures: these different phases have distinctly different mechanical characteristics giving rise to a shape change and accompanied by superelasticity. The superelastic properties of SMA are discussed in Shape Memory Materials, K Otsuka et al. eds. (Cambridge University Press 1998), pp. 27 et seq.
TiNi (generic name is Nitinol) shape memory alloy is commonly used in aerospace, medicine, and consumer products. Thin film SMA has been made in laboratory quantities since 1989. Both bulk and thin film forms are polycrystalline. Single crystal SMA, having a martensitic phase transformation, has much greater superelasticity than polycrystalline SMA. Some SMAs such as Cu—Al—Ni can be made in single crystal form as they are pulled from melt and quenched. These crystals are not limited in length except for the capacity of the machine. TiNi is difficult to grow in this way, and single crystals are generally only a fraction of a centimeter long.
Thin film TiNi is formed by vacuum sputter deposition. A process of crystallization consists of heating to near 500 degrees C., at which temperature many small crystals originate and coalesce into a crystalline form which has the shape memory property.
Single crystals of bulk material are formed by pulling molten material by surface tension to produce a moving crystallization front that progresses through the crystal as it is pulled. The Czochralski method for Silicon and Stepanov method for alloys are examples. These processes work by transforming the molten material (alloy or silicon) from a completely un-structured atomic arrangement to one that is completely ordered as it solidifies. Since in the initial state there are no grain boundaries to interfere with growth of the crystal, it grows as a single crystal in a preferred direction (with a crystallographic index value of <100> for Cu—Al—Ni).
Thin films of alloys, particularly TiNi, are formed as an amorphous layer which has no atomic order. Heating to an elevated temperature (490-520 degree C. for TiNi) allows crystals to form and to grow. When heating is uniform, especially if heating (and cooling) is slow, many nano-crystals form and grow simultaneously. However, by analogy with crystals pulled from melt, if a narrow crystallization front is moved through the thin film expanse by a process that rapidly heats and cools the material, it may be expected to crystallize along a preferred direction and form a single crystal in the same manner as a material pulled from melt.
Another method of forming a single crystal thin film is by use of a substrate with appropriate lattice parameters to grow a crystal epitaxially. The vertical structure will permit larger deformations in the vertical direction and may be appropriate for nano memory applications.
Other methods and processes may be employed to enhance the ability of thin film to grow as a single crystal. In particular, a ‘Fiber’ texture may be induced in the substrate, and ion beam assisted deposition has been successfully used with other thin films to achieve epitaxial growth.
Nitinol materials are used in numerous applications. Superelastic eyeglass frames and components have been known for years, and they are a major selling item in eyeglass manufacturing and retail. Such materials are polycrystalline, and hence the maximum recoverable strain is not achieved because not all crystal domains are oriented in a favorable direction. Superelastic Nitinol is also used in intravascular medical intervention.